Ultrasaurus is a large sauropod from the Jurassic period that lived in South Korea. The holotype specimen of Ultrasaurus turned out to be a mixture of Supersaurus and Brachiosaurus, meaning it was not even a true genus. This is similar to the mix up involving Camarasaurus, Apatosaurus, and the made up genus Brontosaurus.
More real-life information: 'Ultrasaurus on Wikipedia
Jurassic Park Edit
Like Psittacosaurus, Ultrasaurus does not appear in any of the three JP films, but Ultrasaurus can be seen at the beginning of the Jurassic Park: River Adventure at the Universal theme parks. It also had a model of Brachiosaurus too.